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Volcanoes
by Jacaranda Lopez
Submitted to Instructor Hutchinson
GE101 Fall 2003
Introduction
A volcano is a location on the surface of the Earth where magma has erupted out of the
interior of the planet. Magma is molten rock, which has melted from the extreme heat (2200C to
5000C) and pressure that exists inside the Earth. Once molten rock has erupted onto the Earth’s
surface, it is called lava. As lava spreads out on the surface, it cools and becomes solid rock
again. The buildup of lava forms hills and mountains on land, and it forms islands if the volcano
began on the ocean floor. The Hawaiian Islands, for example, exist because of volcanoes.
Stages of Volcanic Activity
Volcanoes are classified as active or inactive. Inactive volcanoes are older and have
usually erupted many times. A volcano is described as active if it is currently erupting or
expected to erupt eventually.
Eruption Stage: A volcanic eruption occurs when lava, gasses, and other subterranean
matter come out of the ground. The exact location of where they come out of is called a vent. A
volcano usually has more than one vent. The following table lists three volcanoes and the date of
the last eruption:
Eruptions can be violent or quiet. Some eruptions send lava high above the surface in
spectacular fountain shapes. Violent eruptions such as these often include chunks of solid rock
that were blown off the interior walls of the vent. Quiet eruptions consist of lava simply flowing
out of vents. Both types of eruptions can also have gasses, such as sulfur and fluorine gas,
coming out of vents.
Cooling and Inactive Stage: Even after a volcano stops erupting, gasses and vapors are
still released. Some volcanoes, however, can end one period of eruptions and begin another after
many years. Eventually, the volcano reaches the cooling stage. While the volcano cools, it
reduces in size from water and weather erosion.
Types of Volcanoes
The different types of volcanoes depend on the volcano’s eruptions. These types vary in
structure. The following chart lists three different types of volcanoes:
Name
Izalco
Kilauea
Mt. Egmont
Country
El Salvador
United States
New Zealand
Volcano Type
Cinder Cone
Shield
Composite
Cinder Cones: Cinder cone volcanoes are formed from explosive eruptions. Because the
materials are ejected high into the air from the violent eruption, they cool before they hit the
ground. Any tiny, fine-grained rock is then blown away by winds. The coarser rock fragments
are left behind in a cone shaped pile, which can be hundreds of meters tall.
Shield Volcanoes: Formed by frequent, quiet eruptions, shield volcanoes are much larger
in width than in height. As smooth lava flows build up, a dome shape is formed. Shield
volcanoes usually change shape when eruptions become explosive late in the life of the volcano.
Composite Volcanoes: Composite volcanoes are very large and are formed from
alternating explosive and quiet eruptions. This results in layers of ejected material covered by the
smooth lava flows. Composite volcanoes are usually symmetrical in shape and can be as high as
several kilometers.
Types of Lava Rocks
Basalt: Basalt (pronounced buh-SALT) is rock composed of mostly feldspar and
pyroxene. Dark in color, basalt is considered to be a fine-grained rock. There are many varieties
of basalt, and some contain iron, magnesium, silica, or aluminum.
Obsidian: Sometimes called volcanic glass, obsidian (pronounced ub-SID-ee-en) is semitranslucent glass that contains a large amount of silicon. Obsidian is usually black or dark gray in
color, and occasionally red or brown. Obsidian that is green is rare, but does exist. When lava
cools so quickly that it does not have time to crystallize, obsidian is formed.
Andesite: Like basalt, andesite (pronounced AN-deh-site) is composed of feldspar and
pyroxene and is a fine-grained rock. However, andesite is usually light to medium gray in color.
Andesite is one of the most common volcanic rocks and can contain olivine, a green mineral.
Conclusion
There are many shapes and types of volcanoes. The shape and type of a volcano is
dependent on the type of lava, the conditions in the Earth, and the age of the volcano. Eruption
can be violent or quiet, frequent or not, and definitely unpredictable.